This invention relates generally to sonar systems, and more particularly to signal processing within high resolution systems so as to form directional beams.
Prior beamforming techniques have employed phase shifting and amplitude shading of signals from a plurality of array sensors, followed by summing of the phase shifted and shaded signals to produce a directional response pattern, or beam. These techniques have employed both analog and digital implementations. However, the requirement to perform numerous multiplications on sampled data during short pulse lengths has restricted the high resolution beamforming process to parallel analog techniques having other disadvantages. For example, analog beamformers, such as that disclosed in U.S. Pat. No. 3,274,536 of F. R. Abbott, et al, have been devised wherein signals from hydrophone elements are divided into sine and cosine quadrature components. These are amplitude shaded in accordance with hydrophone positions, and then summed to provide the desired beamwidth and sidehole suppression. Implementation has been by means of transformer windings wherein shading coefficients are determined by number and direction of winding turns. In sonar applications wherein 360.degree., narrow beam coverage is desired, with the formation of multiple beams during a given pulse length so that the sonar can "look" in more than one angular direction at a given range increment, the number of transformer winding leads and connections becomes very large. Such systems are bulky, expensive, and subject to reliability and maintenance problems.
A digital beamformer has been proposed heretofore which uses quadrature sampled data that is quantized in nonlinear steps. By using geometric encoding of data, multiplication operations become additions of geometric data, with both phase shifting and amplitude weighting being accomplished before the geometrically encoded data is converted to its linear representation for summation in a linear accumulator. In that system, multiple beams are formed serially in one beamformer, with hydrophone element samples serially shifted past one phase shifting and amplitude shading operator. Disadvantages of that system included the added error introduced by geometric quantization and the relative slowness of the beam formation because of the serial element shading technique of the processor.